Multiturn split inductor



N. G. GAGLIARDI Err AL 2,481,008

MULTI-.TURN SPLIT INDUCTOR Sept. 6, 1949.

3 sheets-sheet 1 Filed June 27, 1945 Sept 6, 1949. N. ca. GAGLaARDa E-r AL 2,481,068

MULTITURN SPLIT IIIDUCTOP` 'Filed June 27, 1945 a sheets-snee); z

Sept. 6, 1949. N. G. GAGLIARDI Er Al.V Zfi v MULTITURN SPLIT INDUCTOR Filed June 27, 1945 s shams-sheet s e m1 r www llulmnnw/M Patented Sept. 6, 1949 UNITED STATES PATENT OFFICE MULTITURN SPLIT INDUCTOR York Application June 27, 1945, Serial No. 601,812

8 Claims. 1

This invention relates to induction heating apparatus and more particularly to an improved split forni of inductor or coil having .a lplurality ci turns for applying a high frequency lield'tc an object to vbe heated.

In the induction heating `of articles such as crankshaits or other objects which .have a neck portion accompanied hy enlarged portions vat either :end .of the neck, it is generally .necessary to use a split type .of lc oil, that is, one which may be separated for example, `*substalfltially along a diameter 'to Lailord free `access to vthe space `within the ainductor .for vinserting and re moving the object to he heated. Heretofore it has been the `common practice to .heat obiects such fas crankshafts by yusing such a split coil having a single turn. In 4many cases, however, with .such -a single turn .the distribution of .the resulting high frequency magnetic flux iis such that if the field is applied for a period long enough and is :of ysufficient intensity Vto heat the metal within -theend portions of the ,crankpin to the desired depth, then `the central :portions may become .heated to an excessive depth. Accordlngly, upon quenching, the middle aportion of the crankpin Amay be :hardened to an excessive depth, or if that is avoided by :reducing vthe time and intensity of the heating, then Athe VVend portions may be insufficiently heated for .proper .hardening. .As -willlater be .explained in further detail, this undesirable condition :may be avoided .by using an inductor having two or more turns, whereby the .magnetic field and lresulting .heating may be more uniformly distributed yalong the crankpin portion to lbe heated.

However, there .are a .number of serious problems involved in constructing a practical form of multi-turn inductor, useful :for `purposes such as heating .relatively small erankshafts. T-hat is, the inductor must -not Ionly he of the split form capableof .beingopened up, but at the same time, despite the very limited space available, the convolutions must be :formed with large Asurface areas capable of carrying `-into the small space the required .amount of :high frequency current `for rapid heating .and with avoidance of any lexcessive or irregular induction effects which would prevent flow .of the heavy current uniiornily to the desired in uctor surfaces. Furthermore, provision must ordinarily be made for conducting cooling fluid through. cavities within all -of the conductive parts .of the inductor. This involves problems due to the confined space available and the necessity of having the :inductcr of split form and preferably of a form wherein the .cooling fluid flow maybe :continued uninterrupted lduring the opening and .closing .of the .device for insertion and removal of objects :to lbe heated. Added to athese difficulties is that of 'providing means for rapidly iuniformly .applying -.quenching fluid to .the heated surface of the objects being treated.

The .present invention provides an inductor construction in which the above noted problems have been satisfactorily overcome in various ways so Aas to provide a dependable, compact andihighly efficient cevice for the intended purposes.

Various further and vmore specific objects, fea tures and advantages .of the invention will more clearly appear from the detailed `description given below taken :in .connection with the accompany-- ing drawings which form a part of this specilication and illustrate merely by way of example, a ,preferred forni of the invention. The invenr tion consists in such novel features, arrangements .and combinations of parts as may be shown and described in connection with the apparatus herein disclosed.

in the drawings:

1 is a plan View of a preferred form of construction in accordance with the invention;

Fig. .2 is a schematic diagram for illustrating certain principles of operation of inductors of the class herein referred to, and also illustrating certain possible .modifications of parts of the device of Fig. 1;

Fig. 3 is `a sectional view taken substantially along line 3-3 of Fig. 1;

Fig. 4 is a side elevational view of the device .of Fig. 1;

Fig. 5 is an enlarged View of certain details of the device of Fig. 1;

Figs. 6 and 7 are somewhat diagrammatic plan views of an alternative embodiment of the invention;

Fig. 8 is an edge view of such alternative embodiment; and

Fig. 9 is a perspective view showing portions of such embodiment.

Referring now to the drawings in further detail and more particularly to Figs. 1 and 3, the inductor there shown is formed with two turns, each of which Substantially surrounds a central open space I0 adapted to receive an object to 'be heated, such as the pin portion of a crankshaft. One of these turns, starting at a terminal piece I I, may comprise a sector-like plate I2 extending over an angle of some a second sector-like plate I3 extending ovei1 an angle, for example, of about and a third sectordike plate I4 extending over an angle of some 90. The inner edges of these plates outline the space II!) ywithin which the object to be vheated is placed. In the particular example shown, this space is cylindrical to accommodate a cylindrical portion to be heated, but of course if the cross section .of the object .to be heated is of non-circular, or of Varying diameter, the .inner edges of the plates would be shaped accordingly so that such edges would be -brought into close inductive relation with the surface of the object. The outer or peripheral edges of the platesY I2, I3 and Ic may all be recessed as at I (Fig. 3) and fixed as by a series of screws IS, to a three-part insulation supporting ring I'I, I7', I1", the parts being separable along a line I8 which is substantially along a diameter of the space Ill. Parts I! and II are separated by the terminal piece II. When the device is separated along line I8 for insertion or removal of an object to be heated, it will be apparent that the generally semi-circular plate I3 will be carried by insulation piece I'I', whereas 'sector plates I2 and I4 will both remain in place on the part of the insulation structure designated as I1 and I7.

The second turn of the inductor may be also formed of three sector-like plates I2', IS and I4', located respectively to the rear of the plates I2, I3 and I4 as seen in Fig. 1 and extending in general over corresponding areas (except for connection areas later referred to), and within a plane closely spaced to the rear of the plane of plates I2, I3 and I@ (see Figs. 3-5).

In order to provide for the terminal connections for plates I2 and III' and for connection means between plates I i and I2 to complete the two turns, the shapes of plates I 2 and I2 are made somewhat dilerent adjacent the terminals, as are also the shapes of plates Id and Iii', as will now be described. As best shown in Figs. 1 and 5, plate I4 may be formed with a triangular protruding area as at 22 adapted to contact with and overlie a triangular insert piece 2! (Fig. 5) which in turn contacts with and overlies a triangular extension 22 formed on plate I2'. If desired of course, the piece 2l might be made integral with one of the plates Il! or I2'. The contacting surfaces of the portions 20, 2| and 22 may be secured together as by pins 23. The plate I2, as shown in Fig. l, may be formed with a triangular cuteout portion as at 24 to alford clearance for the portions and 2l. Also, as shown in Fig. 1, the upper righthand edge portion of the plate I4 may be formed with a cut-out area 25 to afford clearance for that part of the plate I2 which is attached to the terminal piece II. Similarly, it will be understood that the underneath plate I2 may be formed with a cut-out portion as shown in dotted lines at 25 in Fig. l, affording clearance for that portion of plate I Il which is attached to a return connection or lug portion II'. Thus, within an angle of a few degrees within the middle upper portion of the plate assembly, provision is made both for connection of the terminals for the inductor and for connecting one inductor turn to the other, all in a compact arrangement such that despite these connections, the whole structure may have a pancake-like form with relatively large unobstructed areas on the `faces of the plates, making it possible to heat objects such as cranksha'fts of a wide variety of shapes or dimensions while still aording ample clearance. The triangular or wedged shaped form of the connection portions 20, 2| and 22 makes it possible to provide conducting surfaces of considerable size as close as possible to the work piece in aperture It for interconnecting the two turns, while still leaving this aperture completely surrounded by inductor surfaces except for a single narrow gap located at the points of the triangular areas. It will be apparent that inductors having more than two turns may be constructed and connected in ac- Y cordance with the same principles.

As indicated in Fig. 3, insulation structure may i be provided at the rear surface of the periphery of the device, with parts corresponding to the parts I1, I l', etc., as on the front side, one of these parts being indicated at Ila in Fig. 3. Insulation spacer means as at 25 may be secured as by screw means 25 between the peripheries of the iront and rear `annular insulation structures. T e structure is such as to leave an annular space as at 27 surrounding the plate assemblies and communicating with space 28 between the two turns of the inductor. From Fig. 3 it is apparent that space 28 opens into the central space I0. As shown in Fig. l, inlet connections as at 29 and 3o may be provided at two or more points for the annular cavity 2l, whereby quenching fluid may be introduced to flow around within the peripheries of the two separable halves of the device and into the space 28 to impinge upon the heated object within space I0. If the device is suspended for use in a vertical plane, then a drip opening as at 3i (Fig. 1) should be provided 'for allowing the quenching fluid to drain from cavity 27.

The upper half of the device as shown in Fig. 1 may be permanently or semi-permanently attached at its insulation periphery, to a suitable supporting bracket as vat 32, the latter in turn being mounted upon an insulation panel 33. The lower half of the device may be detachably clamped to the `upper half by quick-detachable clamping means as at 34, 35 (Figs. l and 4). Each of these clamping means may comprise a member such as at 36 pivoted as at 31 between a pair of small plates S3 fixed to the insulation structure of the upper half of the device. The member 36 ,il is movable arcuately between a pair of similar small plates 39 xed to the lower half of the device. The lower end or shank portion of the member 36 may be threaded to receive suitable stop nut means d. rIihis shank portion may also be surrounded by a spring as at 4I retained within a cylindrical nger piece 42 in a manner such, as will be apparent from the drawings, that one may grasp the piece 112 and pull same downwardly against the action of spring 4I and outwardly to quickly release the clamping means when it is decided to remove the lower half of the device.

The edges of adjacent sector plates along the line I8 may preferably be spaced slightly except as at I8 at regions close to the object to be heated, thus insuring that especially good contact will be made at these regions where it is important to have the ow of high frequency current as free from irregularities as possible. Pairs of pins as at 43, 44 may be provided in suitable apertures at the edges of the upper and lower sector plates to insure proper alignment of the two halves of the device when placed in position and clamped together.

As above indicated, the electrical connections to the sector plates may include terminal pieces II and II which may be secured respectively as by brazing, to sector plates I2 and I4. The

members II and II may be secured also as byv brazing to bracket portions 46 and il respectively and these in turn may be respectively similarly secured to additional bracket portions 43 and 49 extending back through an aperture 50 in the panel 33 and to suitable terminal portions as at 5I, 52 for connection to the source of high frequency current. As best shown in Fig. l, portions I3 and 49 may be interconnected by an insulation piece 53 which will serve to rigidly retain the bracket portions for the incoming and outgoing circuit connections in the desired closely spaced relation. The member 53 as attached to the bracket portions may also serve to supplemen-t the connection lportions 2li, 2i and 22 in rigidly retaining the sector plates of the upper half of Ythe device in proper relative positions. As shown in Fig. .1, the several inductor plates may be formed with apertures lfor receiving in- Sula-tion pins k55 passing through the plates from one face of the device to the other, and embraced by spacer washers 56 between the plates of the two ,turns, to strengthen the assembly and retain the plates nrmly in position.

As shown in Figs. 1, 43 and 4, the various terminal connection portions may be formed with cooling iluid cavities as at 60, EI, and as further shown, cavity t8 for example may communicate with cavities in `all of the sector plates for conducting the cooling fluid therethrough in succession and back to the cavity .6L The cavities in plate I2, -for example, may include a radially extending cavity portion 6,2, the outer end of which communicates with the part of the cavity 60 formed in member II, and the inner end of which communicates with Aan arcuate cavity G3. The other end of cavity 83 4may communicate with .another radially extending cavity portion E4 and the .outer end of the latter cavity may communicate with la tube E extending out through the face of the plate for connection by a exible hose such as the one shown at te (Fig. 4) for conducting the cooling fluid to the cavities in another sector plate. The cavities in all of the plates I2, I2', I4, I4 may be generally similar as will be apparent or readily understood from the dotted lines in Fig. 1. The radial portions of these cavities may be formed by drilling, and such of the openings or the vdrill holes as are unnecessary in the completed structure, may be stopped by brazing suitable plugs in place. The

arcuate portions of these -cavities may be initially milled as open grooves, which maybe later covered over Iby arcuate strips as at 63 (see Fig. 3) these arcuate strips being brazed in place. Thus after the cavities are formed with their end portions suitably plugged, except for the desired inlet and outlet, and with the arcuate strips brazed in place, each of the sector plates will present the external appearance of a solid plate with smooth external yfaces free of any protruding parts which might interfere with proper clearance for lthe work piece being treated in the device. The cooling fluid cavities in sector plates I3 and I3 may, for example, comprise two arcuate `portions as at li), "Il, the cooling fluid entering through a tube 'l2 and then passing through ya radial cavity portion I3 into arcuate cavity yportion "I0, ythence back through arcuate cavity portion II into another radial cavity portion 'I to an outlet tube 15.

rThe inlet and `outlet tubes for the cavities of the several -sector plates may be connected in series or otherwise in various ways by the hose connections. One suitable arrangement of such hose connection is indicated in Fig. 1, wherein the yheavy dot-dash lines indicate hose connections on the front side of the device and the heavy dotted lines indicate the remaining hose connec tions at the rear face of the device. It will be noted that these flexible connections are such as to permit the cooling fluid to ow in series through the cavities of all the sector plates even when the lower half of the device is unclamped from the upper half during insertion or removal of awork piece.

Reference will nowf'be had to Fig. 2 for explaining the l.character :of results obtainable with ya two-turn inductor of this type as contrasted with the results obtainable with a single turn inductor. As shown in this gure, portions a and -b respectively, represent sectional views of two vinductor turns made of plate-like form in :accordance with the invention. Portions of .a fcrank shaft are indicated at c, the two turns being shown in proper spaced inductive relationship to the crankpin portion. If, with two turns thus arranged, the crankpin portion is subjected to induction heating ,only for a brief interval, the heat pattern within the crankpin will be in accordance with that shown by the darker crosshatched areas 8D. That is, two separate annular areas adjacent and .along the two turns respectively will be heated to a shallow depth. However, upon further heating, these two areas will merge and provide a heat pattern .as indicated by the lighter cross-hatched area 8l. That is, the `metal will be Aheated to a desirable .depth almost uniformly along the length of the crankpin except for the end portions Where less heating is ordinarily desired. On the other hand, if an `inductor having but a single central turn is used instead of the two-turn inductor shown, then if the end portions .of the crankp'in are heated to the desired depthy the central portions will be heated too deeply, for example, to an extent indicated by the dotted line vB2 in Fig. 2.

In the various iigures as shown in full lines in Fig. 2, the internal edges of the turns formed by the inductor plates are shaped to extend parallel to the surface to be heated, viz., as shown, these edges form a cylindrical .space concentric with the cylindrical surface of the crankpin. However, in case heat patterns other than that indicated at 8! are desired, the internal edges of the turns may be formed with other shapes. For example, if greater heating at the middle portion of the work piece is desired, then the internal faces of the turns may be shaped as shown by the dotted lines 83. On the other hand, if greater heating is desired at the ends, the ing. ternal shape of the inductor portions may be made to conform with the dotted lines 84 for example. In some cases it may be desired to heat one end of the crankpin or other object more than the other. In that event, for example, the aperture within one of the turns may be made larger than the other and as is indicated by the dotted line 85.

Another form of the invention is shown somewhat diagrammatically in Figs. 'L8 inclusive. In this form, instead of using sector plates as at I2 and I4 as of Fig. 1, one may use plates as at I2a and Ida, each of which extends aro-und the central aperture Iii far enough so that the remaining part of the coil turn may be iiled in by a sector plate I3a, considerably smaller than the plate I3 of Fig. 1.

As indicated in Fig. 8, the second turn may be made up of sector plates as at |21), Mb, and I3b.

It will be understood that for simplicity of illustration, and to avoid repetition, the insulation supporting structure, as well as the electrical connections and cooling fluid connections, have been omitted in Figs. 6-9, but these may be provided in accordance' with the principles disclosed in connection with Figs. 1 4. It will also be understood that the lower portions of the plates I2a, Ila and IZb, |41) which have been cut away in Figs. 6 and 7, may be made in accordance with the construction of the plates I2, I4 and I2', I4',

7 at the areas where the electrical and iluid coni nections are applied.

As further indicated in Figs. 6-9, the edges of the sector plates i311 and |3b may be recessed as shown at 10, 'Il and 10', H to engage complementary recessed edges of the other plates. This makes it possible for the plates i3d, ISb (as held. together by any desired insulation structure) to be conveniently slid out of place when, for example, a crank pin is to be inserted in the aperture I and then quickly slid into place without the necessity of using any hinge or clamping structure for the plates 13a, |31). Also since these plates are relatively small and are slidable into good heat-conductive relation with the other plates, it will usually be unnecessary to provide any cooling fluid conduits therethrough, thus eliminating the problems and diiculties of using iiexible fluid connections to the removable part of the assembly.

While the invention has been described in detail with respect to a particular preferred example, it will be understood by those skilled in the art after understanding the invention, that various changes and further modifications may be made without departing from the spirit and scope of the invention, and it is intended therefore in the appended claims to cover all such changes and modications.

What is claimed as new and desired to beisecured by Letters Patent is:

1. A split two-turn inductor for high frequency induction heating, comprising a plurality of generally sector-shaped metal plates arranged in one plane to form a conductor turn for surrounding an object to be heated at a central cavity therein except for a narrow gap between edges of two of the plates, another plurality of generally similar metal plates arranged in an adjacent parallel plane to form a second conductor turn closely spaced from the first and for also surrounding the object to be heated except at a corresponding gap adjacent said rst-mentioned gap, means at said gaps for `connecting an area at one of said plate edges to an area at an edge of one of said second-named plates, whereby the plates as thus connected form a conductor of two turns for respectively surrounding two parallel bands on the object to be heated, an insulation structure for retaining the plate assemblies in position, said structure being separable into parts, each part carrying plates forming a portion of each conductor turn whereby when such parts are separated, free access may be had to the central cavity for inserting or removing the object to be heated, and means for removably retaining said parts together, the inner edge faces of said plates at said cavity being substantially perpendicular to the planes of said plates.

2. A split two-turn inductor for high frequency induction heating, comprising a plurality of generally sector-shaped metal plates arranged in one plane to form a conductor turn for surrounding an object to be heated at a central cavity therein except for a narrow ygap between edges of two of the plates, another plurality of generally similar metal plates arranged in an adjacent parallel plane to form a second conductor turn closely spaced from the rst and for also surrounding the object to be heated except at a corresponding gap adjacent said first-mentioned gap, means at said gaps for connecting an area at one of said plate edges to an area at an edge of one of said second-named plates, whereby the plates Aas thus connected form a conductor of two turns for respectively surrounding two parallel bands on the object to be heated, an insulation structure for retaining the plate assemblies in position, said structure being separable into parts, each part carrying plates forming a portion of each :conductor turn whereby when such parts are separated, free access may be had to the central cavity for inserting or removing the object to be heated, and means for removably retaining said parts together, the inner edges of said plates at said cavity being of cross sections such that they will conform substantially to a convex surface of revolution.

3. A split two-turn indu'ctor for high frequency induction heating, comprising a plurality of generally sector-shaped metal plates arranged in one plane to form a conductor turn for surrounding an object to be heated at a central cavity therein except for a narrow gap between edges of two of the plates, another plurality of generally similar metal plates arranged in an adjacent parallel plane to form a second conductor turn closely spaced from the rst and for also surrounding the object to be heated except at a corresponding gap adjacent said iirst-mentioned gap, means at said gaps for connecting an area at one of said plate edges to an area at an edge of one of said second-named plates, whereby the plates as thus connected form a conductor of two turns for respectively surrounding two parallel bands on the object to be heated, an insulation structure for retaining the plate assemblies in position, said structure being separable into parts, each part carrying plates forming a portion of each conductor turn whereby when such parts are separated, ree access may be had to the central cavity for inserting or removing the object to be heated, and means for removably retaining said parts together, the inner edges of said plates at said cavity being of cross sections such that they will conform substantially to a concave surface of revolution.

4. A split two-turn inductor for high frequency induction heating, comprising a plurality of generally sector-shaped metal plates arranged in one plane to form a conductor turn, there being a, narrow gap between the edges of two of said plates whereby an object to be heated is surrounded by the conductor turn formed by said plates except for said narrow gap, another plurality of generally similar metal plates arranged in an adjacent parallel plane to form a second conductor turn closely spaced from the first, there being also a narrow gap between the edges of two of the plates forming the second conductor turn, whereby the object to be heated is surrounded except at the corresponding gap adjacent said rst mentioned gap, an electric conductor for connecting an area at one of said plate edges of the rst named plates to an area at an edge of one of said second named plates, whereby the plates, together with said electric conductor, coniointly form a continuous twoturn inductor consisting of two series connected plate like annularly arranged parallel conductors for surrounding the object to be heated, an insulation structure surrounding the rst mentioned plate assembly, an insulation structure surrounding the second mentioned plate assembly, said insulation structures being separable into parts, each part carrying one or more plates forming a portion of its respective conductor turn, mechanism for associating said insulating structures for retaining the plate assemblies in their desired relative positions closely spaced from one another, said parts, when separated, providing free access to the space within the turn for inserting or removing an object to be heated, and mechanism for detachably clamping together said parts.

5. A split two-turn inductor for high frequency induction heating, comprising a plurality of generally sector-shaped metal plates arranged in one plane to form a conductor turn, there being a narrow gap between the edges of two of the plates whereby said conductor turn is adapted for surrounding an open space except for said narrow gap between the edges of said two plates, and whereby an object in said space is surrounded, except for said narrow gap, another plurality of generally similar metal plates arranged in an adjacent parallel plane to form a second conductor turn closely spaced from the first and having a corresponding gap between two of the plates thereof adjacent said iirst mentioned gap, whereby the object to be heated is surrounded thereby except for the corresponding gap, an electric conductor mounted at said gaps for connecting an area at one of said plate edges to an area at an edge of one of said second named plates, the plates together with said electric conductors conjointly `forming a continuous two-turn. conductor for surrounding the object to be heated, an insulation structure surrounding the rst mentioned plurality of plates, an insulation structure for surrounding the second mentioned plurality of plates, each of said structures being separable into parts, each part carrying a portion of each conductor turn, free access being provided to the space within said conductor turns upon separation of said parts for inserting or removing an object to be heated, means for detachably clamping said parts together, said plates each being formed with cooling uid cavities, and flexible insulation conduits for interconnecting said cavities for the flow of cooling fluid therethrough, said insulation structure also being formed with cavities directed toward the space between the two conductor turns,

whereby quenching fluid can be directed into the -f space between the two turns and onto the object being heated.

6. A split two-turn inductor for high frequency induction heating, comprising a plurality of generally sector-shaped metal plates arranged in one plane to form a conductor turn, there being a narrow gap between the edges of two of said plates whereby an open space is surrounded by said plates except for said narrow gap, said open space being adapted to receive an object to be heated, another plurality of generally similar metal plates arranged in an adjacent parallel plane to form a second conductor turn. there being a gap between two of the plates thereof corresponding to the first mentioned gap and adjacent thereto, the second conductor turn thus being adapted for surrounding an open space except for said narrow gap, the latter open space also being adapted to receive said object which is to be heated, an electric conductor directly interconnecting an area at one of said plate edges to an adjacent area at an edge of one of said second named plates, the plates together with said electric conductor conjointly forming a continuous two-turn inductor for surrounding the object to be heated, an insulation structure for each of said conductor turns, mechanism for interconnecting said insulation structures for holding the first and second turns closely spaced and substantially parallel to one another, each insulation structure being separable 10 into parts, each part having connected thereto a substantial portion of each conductor turn, a separation of said insulation structure parts affording free access to the space within the turns for inserting or removing an object, and mechanism for detachably clamping said parts together.

7. In a multiturn inductor vfor high frequency induction heating, the combination comprising a plurality of closely spaced substantially parallel conductor turns, each turn being constituted by a plate-like structure having a central aperture for receiving a workpiece, there being a narrow gap extending substantially radially from the aperture in each plate-like structure, the workpiece being surrounded by each conductor turn, except for its respective narrow gap, means for electrically connecting adjacent conductor turns in the region of the gaps comprising substantially wedge-shaped conductor portions, each comprising a part of the inductor structure, the narrow portions of the wedge-shaped conductor portions being directed toward said aperture, said wedgeshaped portions being substantially co-axial and collectively extending in a direction normal to the planes of the plate-like structures.

8. A split two-turn inductor for high frequency induction heating, comprising an assembly of plate-like conductor elements conjointly forming a disc-like structure arranged in one plane to form a conductor turn, there being a narrow gap between the edges of two of said plate-like conductor elements whereby an object to be heated is surrounded by the conductor turn formed by said plates, except for said narrow gap, another assembly of plate-like conductor elements conjointly forming a disc-like structure arranged in one plane to form another conductor turn there being a narrow gap between the edges of two of the plate-like conductor elements thereof whereby the object to be heated also is surrounded by said other conductor turn except for the narrow gap therein, the second conductor turn being mounted in an adjacent parallel plane closely spaced from the rst conductor turn, an electric conductor for connecting an area adjacent one of said gaps upon one plate assembly to an area adjacent the gap upon the other plate assembly whereby the plate assembly together with said electric conductor conjointly form a continuous two-turn conductor Vfor surrounding the object to be heated, each of i said plate assemblies having a slidable tongue vwith groove-retaining means along its sides whereby such piece may be removed and reinserted to perv'init insertion or removal, in generally radial directions, of workpieces. v

NICHOLAS G. GAGLIARDI. ARMIN A. FINGER. ROBERT J. STANTON.

REFERENCES CITED The following references are of record in the ille of this patent:

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